Frequency detector

ABSTRACT

844,561. Frequency discriminators. PHILIPS ELECTRICAL INDUSTRIES Ltd. Oct. 10, 1958 [Oct. 14, 1957], No. 32423/58. Class 40(5). [Also in Group XXXVI] In a frequency detector for the detection of the variation of a characteristic speech frequency situated within a speech sub-band, the speech sub-band, which is selected by means of a filter 4, is supplied to two channels 6, 7 connected in parallel, the channel 6 comprising a differentiating network 8 and a rectifier 9 and the second a rectifier 11, the output voltages from the channel being fed to a ratio meter 13 the output voltage of which is the ratio of the amplitudes of the channel output voltages and a measure of the frequency. Low-pass filters 10 and 12 are included in branches 6, 7 respectively, and, if the input voltage of rectifier 11 is a (t) at a certain instant that to rectifier 9 is #a(t), so that the ratio of the output voltages of filters 10 and 12 is proportional to awdt/adt and therefore to the frequency. In the embodiment shown, the output direct voltages of filters 10, 12 are converted into alternating voltages of frequencies fi f 2 , corresponding in amplitude to the direct voltages, by means of amplitude modulators 15, 16, carrier wave oscillators 17, 18 of frequencies f 1  f 2  and output filters 19, 20, the outputs of the oscillators and filters being connected through attenuators 21, 22 to adjust the strength ratio of the carrierwave oscillations. A ratio meter for use in the detector is shown in Fig. 4. The alternating voltage derived from channel 6 of frequency f 1  and amplitude-modulated by the rectified differentiated input signal is applied to the grid of a pentode 25 through a resistor 23, the alternating voltage derived from channel 7 of frequency f 2  and amplitude modulated by the rectified input signal being applied to the grid through a resistor 26. Slope control is provided for the pentode 25 and the amplitude of oscillations at frequency f 2  at the output of pentode 25 is therefore kept constant, so that the slope of the pentode 25 varies inversely as the amplitude of the input voltage at frequency f 2 . The voltage at frequency fi is selected by a filter 30 and rectified, the output at 38, 39 being a direct voltage proportional to the amplitude ratio between the output voltages of channels 6 and 7. In the embodiment of Fig. 5, the ratio is determined by amplitude limitation instead of slope control. An amplitude limiter comprising rectifiers 45, 46 has an output consisting of a voltage of frequency f 2  modulated by the voltage of frequency f 1  the frequency deviation of said f.m. voltage being proportional to f 1 : f 2 , the amplitude of f 1  &lt; amplitude of f 2 . The limiter output is fed to an amplifier 51 and a filter 52 tuned to fi, the voltage from this filter being then rectified and giving a direct voltage output which varies as the amplitude ratio of the input voltages of frequencies fi and f 2 . Alternatively the ratio meters may be replaced by one in which the output voltages of channels 6, 7 may be converted into voltages of equal frequencies, the output alternating voltage of channel 6 being subsequently shifted 90 degrees in phase and added to the alternating voltage of channel 7 so that the phase of the sum voltage obtained varies in accordance with the desired amplitude ratio.

1961 F. DE JAGER ET AL 2,995,707

FREQUENCY DETECTOR Filed Sept. 29, 1958 2 Sheets-Sheet 1 FIG.1b

INVENTOR FRANK DE JAGER PETRUS JOSEPHUS VAN GERWEN F. DE JAGER ET AL FREQUENCY DETECTOR 2 Sheets-Sheet 2 INVENTOR FRANK DE JAGER PETRUS JOSEPHUS VAN GERWEN BY E u AGENT Aug. 8, 1961 Filed Sept. 29, 1958 United States Patent C) 2,995,707 FREQUENCY DETECTOR Frank de Jager and Petrus Josephus van Gerwen, Eindhoven, Netherlantls, assignors to North American Philips Company, Inc. New York, N.Y., a corporation of Delaware Filed Sept. 29, 1958, Ser. No. 764,073 Claims priority, application Netherlands Oct. 14, 1957 6 Claims. (Cl. 324-79) This invention relates to a frequency detector for detecting the variation of a characteristic speech frequency lying within a speech sub-band, the speech sub-band selected by means of a filter beng supplied to the input of the frequency detector. For various uses, for example speech synthesis or speech analysis, t is of importance to know these characteristic speech frequencies, the socalled formant frequencies, which are determined by the resonant frequencies of the resonant cavities in the oral and nasal cavities and in the pharynx, and the so-called fundamental frequency, which is determined by the vibration frequency of the vocal cords.

During a speech signal the shape of these resonant cavities is gradually changed by muscular rnovements and at the same time they are excited by a plusatory air current in accordance with the fundamental frequency lying in the frequency band of trom about 80 /5. to 300 c./s. so that during a speech signal a number of oscillations of various frequencies are produced which are pulseamplitude modulated. The speech-components associated with the various formant frequencies are located in different frequency sub-bands determined by formant ranges, the first three subbands, for example, beng situated in the frequency bands of about from 300 c./s. to 800 c./s. 800 c./s. to 2000 c./s., 2000 c./s. to 3400 c./s. This is illustrated in FIG. 1a which is a time diagram showing the variation of the oscillations associated with three letter sounds in the formant range from 800 c./s. to 2000 c./s.

When a conventional trequency detector is used for obtaining, from the pulsatorily varying oscillations shown in FIG. 1a, information about the formant frequency lying within this formant range, the frequency variation beng obtained in the usual manner by determining the average number of passages through zero of the produced oscillations, it has been found that considerable practical diflculties occur.

It is found especially that during the small amplitude values whch occur in the pulsatory oscillations, the output voltage of the frequency detector is adversely afiected in a high degree and this phenomenon is due to the fact that at these instants the variation of the oscillations produced has -an uneven nature while the noise and interference voltages whch occur at these instants exert a comparatively great influence. In addition, in the output voltage of the frequency detector abrupt amplitude variations occur owing to the fact that the frequency detector invariably adjusts itself to the frequency component having the largest amplitude similarly to what takes place when a number of frequency-modulated oscillations of difierent strengths are received simultaneously.

It is an object of the present invention to provide a frequency detector of the kind described in the Preamble in which the said difiiculties are obviated by simple means.

According to the invention, the selected speech subband is supplied on the one hand to a network connected in a first chanel whch difierentiates signal frequencies and is succeeded by a rectifier with an associated low-pass filter, and on the other hand to a rectifier with an assocated low-pass filter which are connected in a second channel, the frequency detector also containing a ratio meter which is controlled by the output voltages of the two channels and is used for determining the ratio of the "ice output voltage of the first channel to the output voltage of the second channel, the output voltage of the frequency detector beng taken from the output circuit of the ratio meter.

In order that the invention may readily be carried out, t will now be described more fully, with reference to the accompanying diagrammatic drawings, in which:

FIG. 1 shows a number of wave forms illustrating the frequency detector in accordance with the invention,

FIG. 2 is a block-schematic diagram of the frequency detector in acco-rdance with the invention,

FIG. 3 is a frequency-voltage diagram illustrating the operation of the frequency detector in accordance with the invention,

FIG. 4 shows in detail the circuit arrangement of an embodiment of a ratio meter for use in a frequency detector in accordance with the inventin, while FIG. 5 shows a preferred embodiment of such a ratio meter.

In the frequency detector in accordance with the invention showin in FIG. 2, which is adapted to the detection of the frequency variation of the formaat frequency in the formant range from 800 c./s. to 2000 c./s., the speech signals are derived from a microphone 1 and subsequently, after amplitude equalization in a ditferentiating network 2, amplified in a low-frequency amplifier, 3. By means of a band-pass filter 4 connected to the output circuit of the low-frequency amplifier 3 the oscillations U (t) situated in the formant range from 800 c./s. to 2000 c./s. are selected, whch oscillations may have the variation shown in FIG. 1a.

In order to detect the frequency variation of the formant frequency lying in the formant range between 800 c./s. and 2000 c./s. the oscillations U(t) selected by the band-pass filter 4 are supplied to a frequency detector 5 which is provided with two channels 6 and 7 connected in parallel to the band-pass filter 4. In the channel 6 the signal U(t) is difierentiated in a differentiating network 8, whch may comprise a series capacitance and a parallel resistance and has a time constant of about 10 seconds, so that a signal S(t) is produced the amplitude of which is proportional to the amplitude of the original sgnal U(t) multiplied by the instantaneous frequency w. The variation of the dfierentiated signal S(t) is illustrated in FIG. 1b.

In the channel 6 the differentated sgnalS(t) is supplied to a rectifier 9 with an associated low-pass filter 10, whch may comprise a series-resistance and a parallelcapacitance, while in the channel 7 the original signal U (t) is likewise supplied to a rectifier 11 with an associated low-pass filter 12, this rectifier circuit 11, 12 beng designed sirm'larly to the rectifier circuit 9, 10. The cut-off frequencies of the low-pass filters 10, 12 are chosen so that during the intervals between the letter sounds an output voltage is also produccd at the output of the low-pass filters 10 and 12, these cutol frequencies beng situated, for example, between 10 c./s. and 50 c./s. and beng about 30 c./s. in the embodiment shown. In FIGURES lc and 1d the broken lines show the envel0pes of the oscillations of FIGURES 1a and 1b, the voltages whch are produced at the output circuits of the low-pass filters 10 and 12 beng indicated by the solid curves.

In order to obtain a voltage whch varies with the frequency variation of the formant requency, the output voltages of the lowpass filters 10 and 12 control a ratio meter 13 the desired variation of the formant frequency, whch is illustrated in FIG. 1e, beng -achieved by determining the ratio between the output voltages of channels of an adjustable attenuator 14 connected in the channel 7 S as to precede the rectifier 11.

In the frequency detector describedthere are produced by re'ctification in the rectfiers 9 and 11 and by the subsequent srnootln'ng in the low-pass filters 10 and 12 at the output circuits of these low pass filters 10 and 12 voltages which are equal to the smoothed values of the voltages set up at the outputs of the rectifiers 9 and 11. I-f the amplitude of the input voltage of the rectifier 11 is a(t) at a certain instant, the input voltage of the rectifier 9 is proportional to the product of the amplitude a(t) and of the i11stantaneous frequency, as has been mentioned hereinbefore, so that in mathematical form the output voltages of the low-pass filters 12 and 10 can be represented by the values of the time integrals:

fadt

ftlwdl (II) taken over a period of time which is determined by the time constants of the low-pass filters 10 and 12.

Thus, at the output circuit of the ratio meter 13 an output voltage is produced which is proportional to the qotient of the time integrals (II) and (I):

(III) the time integr-al in the numerator of the quotient (III) showing that in order to determine the formant frequency there is attached to the frequency w occurring at a certan instant a value which is determined by the instantaneous amplitude a(t) of the signal. Thus the output voltage of the frequency detector is determined substantially by the frequencies of the oscillations during the maximum amplitude values of the letter sounds the frequences of which are found to correspond exactly to the be determined by supplyingeach of these direct voltages to an amplifier having a logarithmic amplification characteristic, the output voltages of these amplifiers being compared in a dilerential network so that at the output of this network a voltage is produced which is proportional to the logarithm of the output voltage of the low-pass filter 10 less the logarithm of the output voltage of the low-pass filter 12. Thus, at the output of the differential network a voltage is produced which is proportional to the loganthm of the ratio between the output voltages of the low-pass filters 10 and 12, the reqnired ratio being obtained by supplying the output voltage of the diierential network to an amplifier having an 6X ponential amplification characteristic.

Instead of directly deterrnining the ratio between the direct voltages shown in FIGURES 10 and 1d by the solid curves, it has been found advantageous to convert the output direct voltages of the low-pass filters 10 and 12 into alternating voltages having frequencies f and f which correspond to the direct voltages in amplitude.

For this purpose in the embodiment described the output direct voltages of the low-pass filters 10 and 12 in the channels 6 -arid 7 control amplitude modulators 15 and 16, which may be push-pull modulators, with associated output filters 19 and and carrier-wave oscillators 17 and 18 the frequencies f and f of which are 32 kc./s. and 48 kc./s., respectively. The ratio meter to he employed will be described more fully hereinafter with reference to FIGURES '4and 5.

It can be ensured in a simple manner that even in the absence of a speech signal the ratio meter 13 provides a maan output voltage by only providing that in this event the carrier-wave oscillations f and f of the oscillators 17 and 18 are set up at the input of the ratio meter 13 in a suitable strength ratio. For this purpose the present embodirnnt the output circuits of the carrierwave oscillators 17 and 18 arecoimected, through adjustable attenuators 21 and 22, to the outputs of the output filters 19 and 20, respectively, provision being made by suitable adjustment of the attenuators 21 and 22 that in the absence of a speech signal the ratio meter 33 supplies an output voltage which corresponds to a frequency situated in abut the middle portion of the formant range concerned. Such an adjustnient also provides the advantage that when a speech signal occurs aftera speech interval, the frequency detector rapidly adjusts itself to the desired value.

Obviously the adjustment of the ratio between the carrier frequencies f and f in the absence of a speech signal can also be ensured in a different manner, for example, by a slight modification in the push-pull of the push-pull modulators 15, 16 used.

FIG. 3 shows a voltage diagram of the frequency detector shown in block-schematic form in FIG. 2, the output voltage V of the frequency detector being plotted as a function of the input voltage V at various frequencies of the input voltage.

In the absence of an input voltage there is produced at the output of the frequencydeteotor owing to the adji1stment of the attenuators 21, 22 an output voltage P which, in the embodiment concerned, corresponds to a frequency of abo-ut 1400 c./s.

If there is supplied to the input of the frequency de tctor an oscillation of constant frequency 1, the amplitude of which is gradually increased, sta1ting from the adjusted point P the output voltage associated with this frequency 7 will be reached even at a very low Input voltage while it remains substantially constant ata further increase of the input voltage. Thus, the curves shown represent the variation of the output voltage V of the frequency detector as a function of the input voltage V at frequencies of 800 c./s., 1400 c./s. and 2000 c./s. respectively.

In the frequency detector described it is remarkable thatthe output voltage V of the frequency detector is substantially independent of the amplitude of the input voltage while furthermore, as will be seen from the figure, this input voltage varies in a substantially linear relationship with the frequency within the comparatively wide freciuency band (800 c./s.-2000 o./s.) of this formant range. v'

Tests have also shown that when an mterm1ttent alternating voltage is supplied tothe input, the output voltage of the frequency detector is substantially independent of 'theratio between duratidn and spaciing of these wave 'trains (duty cycle).

FIG. 4 shows in detail the circuit arrangement of a ratio meter for use with the frequency detector shown in jpentode 25, there being also applied to this/control grid through a resistr 26 the alternating voltage f which is derived from the channel 7 and the amplitudeof which is deterrhined by the rectified input signal.

Forratio measurements the anode ciicuit of the pentode 25 contains two parallel-connected restrs 27, 28, there being connected to a tapping on the1esistor 27 a selecting filter 29 tuned to thefi'equeny -f2 and to a rived from the filter 29 is rectified bya rectifier 32 to which a bias voltage is applied which is supplied by a potentiometer 31 connected between the positive voltage terminal and earth, the negative direct voltage obtained being supplied, through a resistor 34, to the control grid of the pentode 25 tot slope control. Owing to this slope control the amplitude of the oscillations at frequency f taken from the output of the pentode 25 Will be kept substantally at a constant level so that the slope of the pentode 25 varies in substantially inverse proportion to the amplitude of the oscillator f supplied to the input of the pentode '25.

The oscillation at frequency f selected by the filter 30 is rectified in a rectifier circuit comprising a rectifier 35 and an output impedance 36 and subsequently is supplied, through a low-pass filter 37, to output terminals 38, 39, the direct voltage taken from the terminals 38, 39 being the output voltage of the ratio meter." The use of the slope control ensures that the oscillations at frequency f are amplified in the pentode 25 in inverse proportion to the amplitude of the oscillation f supplied to the input of the pentode 25, so that the voltage taken from the terminals 38, 39 is proportional to the amplitude ratio between the output voltages of channels 6 and 7.

Sirnilarly to the ratio meter shown in FIG. 4, in the embodiment shown in FIG. the output voltages of the channels 6 and 7 are supplied through series-resistors 40 and 41 and a grid capacitor 42 to the control grid of a pentode 43, the amplitude ratio being determined, however, by amplitude lmitation instead of by slope control.

For this purpose, there is connected to the output circuit of the pentode 43 provided with a resonant circuit 44 which passes the oscillations and an amplitude limiter comprising two rectifiers 45, 46 connected with asymmetric conductivty with respect to each other, there being connected to the output circuit of the limiting arrangement a resonant circuit 47 which passes the oscillations and f While the junction of the rectifiers 45, 46 is connected to the junction of a potentiorneter 49, 50 which is connected between the positive voltage terminal 48 of the voltage supply source and earth and supplied a current in the pass direction of the rectifiers 45, 46.

Bach time the voltage at the anode circuit of the pentode 43 exceeds the voltage at the junction of the rectifiers 45, 46, the rectifier 45 is cut oi, while a constant current flows through the resistor 49 of the potentiometer 49, 50, which is made comparatively large, and through the rectifier 46 to the output impedance 47, while conversely, when the output voltage of the pentode 43 is lower than the voltage at the junction of the rectifiers 45, 46, the rectifier 45 is conductive and the rectifier 46 is cut ol so that no current flows to the output impedance 47. The limiter is proportioned so -as to provide complete lmitation already at signals of small amplitude.

In the arrangement described the output voltage of the limiter circuit is supplied for amplification to a pentode 51, the oscillation f being selected by means of a selecting filter 52 connected in the output circuit of the pentode 51 and providing the output voltage of the frequency detector after rectification in a rectifier stage comprising a rectifier 53 and an output impedance 54 and subsequent smoothing in a low-pass filter 55. The output voltage of the frequency detector is taken from output terminals 56, 57.

If in the circuit arrangement described care is taken to ensure that the amplitude of the oscillation f taken from the channel 6 is smaller than the amplitude of the oscillation f supplied by the channel 'l, there are produced at the output circuit of the limiter arrangement current pulses having a repetition frequency f the duration of which varies in accordance with the amplitude of the oscillation at the frequency f The value of these variations in duration is determined by the amplitude ratio between the oscillations ;f and f so that after selection of the oscillation in the filter 52 and subsequent rectification in the rectifier stage 53, 54 and srho0thing in the low-pass filter 55 there is set up at the output terminals 56, 57 a direct voltage which varies in accordance with the amplitude ratio between the oscillations 31 and supplied to the input oi the pentode 43. In order to achieve a linear relationship between the direct voltage taken irom the output terminals 56, 57 and the amplitude ratio between the oscillations f and f at the input of the pentode 43, the amplitude of the oscillation f is made smaller than the amplitude of the oscillation by a factor of, for example, 4 to 5. It should be remarked here that in the output circuit of the limiter in'stead of the oscillation at frequency f use can also be made of the image frequency of f with respect to 2, which consequently has a frequency of 2f -f The ratio meters described hereinbefore can obviously be replaced by other ratio meters. For example, the output voltages of the low-pass filters in the channels 6 and 7 may be converted into alternating voltages of equal frequencies the output alternating voltage of the channel 6 being subsequently shifted in phase and added to the alternating voltage of the channel 7. Thus the phase of the sum voltage obtained varies in accordance with the amplitude ratio between the output voltage of channel 6 and the output voltage of channel 7 so that the desired ratio is obtained by phase measurement.

As has been mentioned hereinbefore, the frequency detector described can also be used for the detection of the variation of the fundamental frequency, for which purpose a suitable speech sub-band containing the fundamental frequency is supplied to the input of the frequency detector. This speech sub-band can be selected directly from the speech signal or it can be obtaned by means of amplitude detection of preferably at least one higher formant range.

What is claimed is:

1. A frequency detector for the detection of the variation of a characteristic speech frequency situated Within a speech sub-band comprising first and second channels having input and output terminals, means applying signals of said sub-band to the input terminals of said first and second ohannels said first channel comprising in the order named serially connected difierentiating means, first recti dier means, first low-pass filter means, and modulator means, said second channel comprising in the order named serially connected second rectifier means, second lowpass filter means, and second modulator means, ratio meter means connected to said output terminals to provide an output signal that is a function of the ratio of the output voltages of said first and second channels, and oscillator means connected to said first and second modulator means.

2. The frequency detector of claim 1, comprising means for adjusting the outputs of said first and second modulator means so that an output signal trom said ratio meter means occurs in the absence of said signals of said sub-band.

3. A frequency detector comprising a source of input signals, first and second channels having input and output terminals, means applying said signals to said input terminals, said first channel comprising in the order named serially connected diiferentiating means, rectifier means, lowpass filter means, and first modulator means, said second channel comprising in the order named serially connected rectifier means, low-pass filter means, and second modulator means, first and second oscillator means having difierent frequency oscillations connected respectively to said first and second modulator means whereby said oscillations -are modulated by the signals in their respective channels, and ratio meter means connected to said output terminals to provide an output signal that is a function of the ratio of the output voltages of said first and second ol1annels.

4. The frequency detector of claim 3, in which said ratio meter means comprises an amplifier device having an input 'electrode and an output electrode, said output terminals being conneoted to said input electrode, a first output filter tuned to the frequency of oue of s"aid oscillator means, rectifier means c'onneted betweei1 said input eleotrode and said first output filter, and a second output filter tuned to the frequency of the other of said oscillator means and being connected between said output eleotrode and an output circuit.

5. The frequency detector of claim 3, in which said ratio meter means compriss lmiter means having an input circuit connected to said output terminals and an output circuit cofinected to cfreqency seleotive filter means tuned to the frequency of one of said oscilltor means, the amplitude of the output voltage of the (me -cha'nnel comprsing said one oscillator means being less than the output voltage of the other channel.

6. The frequency detector of claim 5, in which said limter compn'ses a pair of rectifiers oonnected with asymmetri concluctvity With respect to each other between.

Refrences Cited in the file of this patent UNITED STATES PATENTS 2418284 Winchel et al. Apr. -1, 1947 2,541,067 Jaynes Feb. 13, 19 51 2,576249 Baruejr NOV. 27, 1951 2,711,516 Fredeudall June 21, 1955 2857,465 Schroeder 0012. 21, 1958 2,896,161 FOX July 21, 1959 FREIGN PATENTS 788,565 Great BI"3.II Jan. 2, 1958 

